Power conversion system with dc-bus pre-charge

ABSTRACT

A power conversion system comprises a plurality of power modules, each including a power input end; a charging input end; a power output end; at least one power conversion unit, each including an AC/DC conversion unit and at least one DC-Bus capacitor and being connected to the power input end and the power output end; and a pre-charging unit connected to the charging input end for receiving direct current and connected to the DC-Bus capacitor. The pre-charging unit starts to charge the DC-Bus capacitor of one of the power modules when said power module breaks down or the load of the power conversion system is light so that no current flows through the AC/DC conversion unit. The power input ends of the power modules are connected in series and then connected to an AC power source, and the power output ends of the power modules are connected in parallel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This non-provisional application is a Continuation-In-Part applicationof patent application Ser. No. 16/423,147 filed in the U.S. on May 27,2019 which claims priority to China Application Serial Number201810708851.X, filed Jul. 2, 2018, the entire contents of which arehereby incorporated by reference.

Some references, if any, which may include patents, patent applications,and various publications, may be cited and discussed in the descriptionof this invention. The citation and/or discussion of such references, ifany, is provided merely to clarify the description of the presentinvention and is not an admission that any such reference is “Prior Art”to the present invention described herein. All references listed, cited,and/or discussed in this specification are incorporated herein byreference in their entireties and to the same extent as if eachreference was individually incorporated by reference.

BACKGROUND OF THE PRESENT INVENTION 1. Field of the Present Invention

The present invention relates to a power conversion system, andparticularly to a power conversion system adapted for medium voltage.

2. Related Art

With the increasing demand for green smart power, voltage levels ofpower electronic converters have been gradually expanding fromconventional and mainstream low-voltage commercial power (110-380 V) tomedium-voltage distribution networks (10-35 kV), such that the powerelectronic devices have been applied to medium-voltage systemsincreasingly. Moreover, with the rapid development of Internet datacenters and the electric vehicle industry, the demand for DC powersupply is increasing. Demands for the diversity of applications and thediversity of system architectures require converters to be designed suchthat they can be easily expanded. As an efficient way to solve thisproblem, a plurality of power electronic converters may be combined inseries or parallel, i.e., so-called combination-typed power converter.However, the pre-charging of DC-Bus capacitors in the combination-typedpower converter becomes a new bottleneck. At present, to inhibit surgeinrush current due to charging of the DC-Bus capacitors when the systemis powered up, it is necessary to pre-charge the DC-Bus capacitors via apre-charging circuit before the combination-typed power converter ispowered up. With a trend that the levels of voltages and currents invarious applications are increasing, it is highly demanded that thepre-charging circuits be designed according to the modular design of thecombination-typed power converters, so as to ensure a compact hardwarestructure, small spatial size and lower circuit losses.

FIG. 1A shows a configuration of a power conversion system in the priorart. As shown in FIG. 1A, the power conversion system in this exampleincludes a plurality of power modules 1 to N. Power input ends of theplurality of power modules 1 to N are cascaded at input side. Poweroutput ends of the plurality of power modules 1 to N are connected inparallel to a DC bus at output side. Moreover, each of the power modules1 to N includes an AC/DC conversion circuit and a DC/DC conversioncircuit connected to the AC/DC conversion circuit. DC-Bus capacitorsC₁₁, C₁₂, C₂₁, C₂₂, . . . , C_(N1), C_(N2), etc., are connected betweenthe two conversion circuits. Here, a pre-charging circuit is formed byrespectively connecting soft-start resistors R_(A), R_(B), and R_(C)with switches S_(A), S_(B), and S_(C) in parallel, as being surroundedby a dashed block in the figure. One end of the pre-charging circuit isconnected in series to phase A, phase B, and phase C of a medium-voltageinput of the system. The other end of the pre-charging circuit isconnected to the power input ends of the power modules, for charging theDC Bus via the AC/DC conversion circuits or the DC/DC conversioncircuits in the main power circuits of the power modules 1 to N.

FIG. 1B shows a process for performing the pre-charging with the powerconversion system shown in FIG. 1A. Specifically, the DC-Bus capacitorsin the power modules 1 to N are charged simultaneously throughsoft-start resistors in respective phases when a switch for the mediumvoltage is closed. After a sufficient voltage is created on the DC-Buscapacitors, i.e., after the charging is completed, switches S_(A),S_(B), and S_(C) in respective phases are switched on so as to bypassrespective soft-start resistors R_(A), R_(B), and R_(C), and thenactivates the main power circuits in respective power modules 1 to N.Taking phase A as an example, the DC-Bus capacitors C₁₁, C₁₂, C₂₁, C₂₂,. . . , C_(N1), C_(N2) are charged simultaneously through the soft-startresistor R_(A) in phase A. After the capacitor voltage of the DC-Buscapacitors C₁₁, C₁₂, C₂₁, C₂₂, . . . , C_(N1), C_(N2) reaches apredetermined value, the switch S_(A) is switched on so as to bypass thecorresponding soft-start resistor R_(A). Then, the power modules 1 to Nmay be activated for converting electrical energy.

However, the above power conversion system in the prior art has thefollowing shortcomings.

1) First, it is limited to the voltage levels of the main power circuitsin the power modules 1 to N. The soft-start resistors R_(A), R_(B), andR_(C), and the switches S_(A), S_(B), and S_(C) are required to bemedium-voltage elements, which are large in size and lead to a highcost. For example, the size of a 10 kV SVG pre-charging circuit may be600 mm*600 mm*600 mm in volume. Further, in control of those switchesS_(A), S_(B), and S_(C), the voltage level isolation at medium voltagehas to be taken into consideration.

2) Second, since the pre-charging circuit is disposed in the main powercircuit, the pre-charging circuit can only be fed with medium-voltagealternating current passively. The power modules 1 to N cannot behot-plugged.

3) Third, since the soft-start resistors R_(A), R_(B), R_(C) areconnected in series to a main power circuit between respective powerinput ends of the power modules 1 to N and respective medium voltageinputs of the system, the entire system may not work when the soft-startresistors R_(A), R_(B), R_(C) or the switches S_(A), S_(B), and S_(C)fails.

4) Fourth, the pre-charging can only be done at one time. If additionalcontrol is not provided, there may be a risk of unbalance in capacitorvoltage.

5) Fifth, when grid voltage changes, the soft-start resistors R_(A),R_(B), R_(C), and the switches S_(A), S_(B), S_(C) have to beredesigned.

FIG. 2 shows a configuration of another power conversion system in theprior art, in which the pre-charging circuits includes a soft-startresistor and a switch connected to the soft-start resistor in parallel.This power conversion system shown in FIG. 2 differs from the powerconversion system shown in FIG. 1A in that the pre-charging circuit isdistributed into respective power modules 1 to N. Specifically, asoft-start resistor is connected in series to an input end of the mainpower circuit in each of the power modules 1 to N, respectively. Aswitch is connected in parallel to the soft-start resistor in each ofthe power modules 1 to N. In this way, pre-charging of the DC-Buscapacitors in respective power modules may be implemented via thedistributed pre-charging circuit. Taking phase A as an example, asoft-start resistor R_(1A) of a power module 1 is connected in series toan AC input end of the power module 1. A switch S_(1A) is connected inparallel to the soft-start resistor R_(1A). A soft-start resistor R_(2A)of a power module 2 is connected in series to an AC input end of thepower module 2. A switch S_(2A) is connected in parallel to thesoft-start resistor R_(2A). Similarly, a soft-start resistor R_(NA) of apower module N is connected in series to an AC input end of the powermodule N. A switch S_(NA) is connected in parallel to the soft-startresistor R_(NA). Moreover, the process for pre-charging in the powerconversion system shown in FIG. 2 is similar to that in FIG. 1B.Specifically, the DC-Bus capacitors in the power modules 1 to N arecharged simultaneously through respective soft-start resistors when theswitch for the medium voltage is closed, i.e., when the medium voltageinput is switched on. After a sufficient voltage is created on theDC-Bus capacitors, i.e., after the charging is completed, the switchesare switched on so as to bypass respective soft-start resistors, therebyactivating the main power circuits in respective power modules 1 to N.Taking phase A as an example, the DC-Bus capacitors C₁₁, C₁₂, C₂₁, C₂₂,. . . , C_(N1), C_(N2) are charged simultaneously through the soft-startresistors R_(A) in phase A. After the capacitor voltage of the DC-Buscapacitors C₁₁, C₁₂, C₂₁, C₂₂, . . . , C_(N1), C_(N2) reaches apredetermined value, the switch S_(A) is switched on so as to bypass thecorresponding soft-start resistor R_(A). Then, the power modules 1 to Nmay be activated for converting electrical energy. Taking phase A as anexample, when the medium voltage in phase A is switched on, the DC-Buscapacitors C₁₁, C₁₂ in the power module 1 are charged simultaneouslythrough the soft-start resistor R_(1A). The DC-Bus capacitors C₂₁, C₂₂in the power module 2 are charged simultaneously through the soft-startresistor R_(2A). After the voltage on the capacitor reaches apredetermined value, the switch S_(1A) is switched on so as to bypassthe corresponding soft-start resistor R_(1A). Similarly, the switchS_(2A) is switched on so as to bypass the corresponding soft-startresistor R_(2A), and so on. Then, the power modules 1 to N are activatedfor converting electrical energy.

However, the above power conversion system in the prior art has thefollowing shortcomings.

1) First, since the soft-start resistors and the switches in thepre-charging circuit are connected in series to the main power circuitsof the power modules 1 to N, the current levels of those soft-startresistors and switches have to be selected depending on the currentlevels of the main power circuits of each power module. Therefore, thosesoft-start resistors and switches have a large volume. For example, thesize of a single switch is not less than 50.5 mm*32.9 mm*36 mm involume. Further, too many switches may increase the cost of the system.

2) Second, since the soft-start resistors and the switches in thepre-charging circuit are connected in series to the main power circuitof the power modules 1 to N, the entire system may not work when somesoft-start resistor or some switch fails.

3) Third, since the pre-charging circuit is disposed in the main powercircuit, the pre-charging circuit can only be fed with medium-voltagealternating current passively.

4) Fourth, the pre-charging can only be done at one time. If additionalcontrol is not provided, there may be a risk of unbalance in capacitorvoltage.

Therefore, it is highly demanded a power conversion system to address atleast some of the deficiencies of the power conversion system describedabove.

SUMMARY OF THE PRESENT INVENTION

In view of the above, it is an aspect of the present invention toprovide a power conversion system to effectively pre-charge DC-Buscapacitors in various power modules and to allow the pre-chargingcircuit to be smaller in size and less in power consumption.

In one aspect, the present invention provides a power conversion systemconnecting to a load, comprising a plurality of power modules. Eachpower module includes a power input end; a charging input end; a poweroutput end; at least one power conversion unit, each of the powerconversion unit including an AC/DC conversion unit and at least oneDC-Bus capacitor and being electrically connected to the power input endand the power output end, wherein the AC/DC conversion unit iselectrically connected between the power input end and the DC-Buscapacitor; and a pre-charging unit electrically connected to thecharging input end for receiving direct current and electricallyconnected to the DC-Bus capacitor for pre-charging the DC-Bus capacitor.The pre-charging unit starts to charge the DC-Bus capacitor of one ofthe plurality of power modules when the one of the plurality of powermodules breaks down or the load of the power conversion system is light.The power input ends of the plurality of power modules are connected inseries and then electrically connected to an AC power source, and thepower output ends of the plurality of power modules are connected inparallel.

Hereinafter, the above description will be described in detail withreference to implementations, and a further explanation of the technicalsolutions of the present invention will be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

To make the above and other objects, features, advantages, and examplesof the present invention more apparent and straightforward, a briefdescription of the drawings is provided as follows:

FIG. 1A is a schematic diagram showing a configuration of a powerconversion system in the prior art;

FIG. 1B is a flow chart showing a process for performing thepre-charging with the power conversion system shown in FIG. 1A;

FIG. 2 is a schematic diagram showing a configuration of another powerconversion system in the prior art;

FIG. 3A is a schematic diagram showing a configuration of a powerconversion system according to one embodiment of the present invention;

FIG. 3B is a flow chart showing a process of pre-charging using thepower conversion system shown in FIG. 3A;

FIGS. 4A-4B are schematic diagrams showing the operation principle ofpre-charging using the power conversion system according to one exampleof the present invention;

FIG. 5A is a preferred embodiment of an auxiliary power source (AuxPower) at input end of the pre-charging unit in each of the powermodules of FIG. 3A;

FIG. 5B is another preferred embodiment of the auxiliary power source(Aux Power) at input end of the pre-charging unit in each of the powermodules of FIG. 3A;

FIG. 5C is yet another preferred embodiment of the auxiliary powersource Aux Power at input end of the pre-charging unit in each of thepower modules of FIG. 3A;

FIG. 6 is a schematic diagram showing a circuit structure of a firstapplication embodiment in which the power conversion system of thepresent invention is applied to a three-level Diode Neutral PointClamped (DNPC) cascaded system for pre-charging;

FIG. 7 is a schematic diagram showing the circuit structure of thepre-charging unit 1 disposed in the power module 1 (PM1) of FIG. 6;

FIG. 8 is a schematic diagram showing a circuit structure of a secondapplication embodiment in which the power conversion system of thepresent invention is applied to a full bridge cascaded system forpre-charging;

FIG. 9 is a schematic diagram showing a circuit structure of a thirdapplication embodiment in which the power conversion system of thepresent invention is applied to a DC/DC full-bridge system forpre-charging;

FIG. 10 is a schematic diagram showing the circuit structure of a fourthapplication embodiment in which the power conversion system of thepresent invention is applied to a three-level DNPC cascaded system forpre-charging;

FIG. 11 is a flow chart of a pre-charging method for DC-Bus capacitorsin a medium-voltage power conversion system according to one embodimentof the present invention.

FIG. 12 is a preferred embodiment of an AC/DC conversion unit in a powerconversion unit in each of the power modules of FIG. 3A.

FIG. 13 is a preferred embodiment of an AC/DC conversion unit, wherein aDC/DC converter is added into the AC/DC conversion unit of FIG. 12.

FIG. 14 is another preferred embodiment of an AC/DC conversion unit in apower conversion unit in each of the power modules of FIG. 3A.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In order to make the description of the present invention more elaborateand complete, reference may be made to the accompanying drawings and thevarious examples described below. Like numbers in the drawings indicatelike components. On the other hand, some known components and steps arenot described in the embodiments to avoid unnecessarily limiting thepresent invention. In addition, some known structures and elements areshown in the drawings schematically to simplify the drawings.

FIG. 3A is a schematic diagram showing a configuration of a powerconversion system according to one embodiment of the present invention.As shown in FIG. 3A, the power conversion system according to theembodiment comprises a plurality of power modules 1 to N (N is apositive integer greater than or equal to 2). The power modules 1 to Nare cascaded-connected at input side and connected in parallel at outputside. Each of the power modules 1 to N includes a power input end, acharging input end, and a power output end. The power input ends of thepower modules 1 to N are connected in series and then electricallyconnected to an AC power source. For example, the power input ends ofthe power modules 1 to N may be connected in series and thenelectrically connected to phase A, phase B, and phase C of amedium-voltage three-phase AC input via an inductor. It is understoodthat this AC power source is not limited to a medium-voltage AC powersource (with a voltage ranging from 10 kV to 35 kV), but can also beother forms of AC power sources, which is not intended to limit thepresent invention. The power output ends of the power modules 1 to N areconnected in parallel to an output DC bus. Of course, it is understoodthat the power output ends of the power modules 1 to N may alternativelyoutput independently, which is not intended to limit the presentinvention, either.

In the embodiments, each power module further includes a powerconversion unit. The power conversion unit is electrically connected tothe power input end and the power output end respectively. The powerconversion unit further includes at least one DC-Bus capacitor. Forexample, in the embodiment shown in FIG. 3A, each of the power modules 1to N further includes a power conversion unit including an AC/DCconversion circuit, a DC/DC conversion circuit, and the like.Specifically, power module 1 includes DC-Bus capacitors C₁₁ and C₁₂. Thepower module 2 includes DC-Bus capacitors C₂₁ and C₂₂. Similarly, thepower module N includes DC-Bus capacitors C_(N1) and C_(N2). It isunderstood that the number and the connection of the DC-Bus capacitorsin each power module are not limited to the number and the connectionshown in FIG. 3A. That is, each power module may include only one DC-Buscapacitor, or include two or more DC-Bus capacitors. These DC-Buscapacitors can be connected in series, or parallel, or serve as DC-Buscapacitors of cascaded conversion units, etc., which are not intended tolimit the present invention.

In the embodiments of the present invention, each power module mayfurther include a pre-charging unit. Therein, the pre-charging unit iselectrically connected to the charging input end to receive directcurrent, and is electrically connected to the DC-Bus capacitors topre-charge the DC-Bus capacitors. Pre-charging refers to a process forcharging a DC-bus capacitor to increase the voltage of the DC-buscapacitor before the power conversion is performed through switchingoperations of the power modules. In the embodiment shown in FIG. 3A, thepower modules 1 to N include pre-charging units 1 to N, respectively.The pre-charging units 1 to N are connected to charging input ends ofthe N power modules, respectively, and are electrically connected to aplurality of auxiliary power sources Aux Power 1 to N, respectively, toreceive direct current supplied from the auxiliary power sources AuxPower 1 to N. Moreover, it is understood that although the auxiliarypower sources Aux Power 1 to N output direct current to the charginginput ends, the auxiliary power sources Aux Power 1 to N may receivefrom any input sources, such as alternating current or direct current.Direct current may be output to the pre-charging units via variouspossible conversion circuits such as AC-DC or DC-DC conversion circuits,which will be described in more detail below.

The specific structure and operation principle of the power conversionsystem according to the embodiment of the present invention will bedescribed in detail below with reference to FIGS. 4A to 4B and FIG. 3B.FIGS. 4A-4B are schematic diagrams showing the operation principle forperforming pre-charging with the power conversion system according tothe embodiment of the present invention. FIGS. 4A-4B also showcomponents included in the power modules of the power conversion systemaccording to the embodiment of the present invention and theirconnection relationships. To simplify the drawings and facilitating thedescription, FIG. 4A only shows the connection between two powermodules, and only shows one DC-Bus capacitor in each power module.However, it is understood that those are not intended to limit thepresent invention.

In the embodiment of the present invention shown in FIG. 4A, the powermodule 1 (PM1) comprises a power conversion unit PCU1 and a pre-chargingunit 1 (PU1). The power conversion unit PCU1 includes a DC-Bus capacitorC11. A DC-Bus capacitor may be formed by connecting a plurality ofcapacitors in parallel, and the present application is not limitedthereto. The power module 1 (PM1) further comprises a power input endPM1-IN, a charging input end PU1-IN, and a power output end PM1-OUT.Similarly, the power module 2 (PM2) comprises a power conversion unitPCU2 and a pre-charging unit 2 (PU2). The power conversion unit PCU2includes a DC-Bus capacitor C₂₁. The power module 2 (PM2) furthercomprises a power input end PM2-IN, a charging input end PU2-IN, and apower output end PM2-OUT.

The power module 1 (PM1) and the power module 2 (PM2) are cascaded at aninput side. That is, a second input terminal IN₁₂ of the power input endPM1-IN of the power module 1 (PM1) is connected in series to a firstinput terminal IN₂₁ of the power input end PM2-IN of the power module 2(PM2). Further, a first input terminal IN₁₁ of the power input endPM1-IN of the power module 1 (PM1) may be connected to phase A/B/C of amedium-voltage input (as shown in FIG. 3A). A second input terminal IN₂₂of the power input end PM2-IN of the power module 2 (PM2) may beconnected to a first input terminal of a power input end of the nextpower module. However, a specific way for connecting the power inputends is not limited thereto. The power output ends PM1-OUT and PM2-OUTof the power module 1 (PM1) and the power module 2 (PM2) may beconnected in parallel or may output independently, or the like. However,the present invention is not limited thereto. Moreover, the pre-chargingcircuit 1 (PU1) and the pre-charging circuit 2 (PU2) are electricallyconnected to respective charging input ends PU1-IN and PU2-INindependently, and connected in parallel to the DC-Bus capacitors C₁₁and C₂₁, respectively. Therefore, the DC-Bus capacitors in respectivepower modules can be pre-charged directly.

By taking a power module PM as an example, FIG. 4B shows a method forpre-charging DC-Bus capacitor C_(DC-Bus) in the power module PM by apre-charging unit in the power module PM according to the embodiment. Inone embodiment, each power module PM further comprises a controller anda voltage sampling unit. The voltage sampling unit is electricallyconnected to the DC-Bus capacitor C_(DC-Bus) to sample a voltage of theDC-Bus capacitor C_(DC-Bus). The controller is electrically connected tothe voltage sampling unit and the pre-charging unit in the power modulePM. As such, the pre-charging unit can be controlled by the controllerto perform a DC/DC voltage converting process on the received directcurrent so as to pre-charge the DC-Bus capacitors C_(DC-Bus). Further,the voltage of the DC-Bus capacitors C_(DC-Bus) can be sampled by thevoltage sampling unit. The sampled values may be sent to the controller.When the voltage of the DC-Bus capacitors C_(DC-Bus) reaches a threshold(i.e., when the charging is completed), the pre-charging unit can becontrolled by the controller to cease the pre-charging.

FIG. 3B is a flow chart showing a process of pre-charging using thepower conversion system shown in FIG. 3A. As shown in FIG. 3B withreference to FIG. 3A, in one embodiment of the present invention, eachof the power modules 1 to N of the power conversion system can chargethe DC-Bus capacitors with the pre-charging units, and then can controlthe pre-charging units by the controller to cease charging aftercompletion of charging. At this point, the main power circuits, such asthe power conversion units, of the power modules 1 to N can be activatedby switching on the medium voltage input. Therefore, the pre-charge ofthe DC-Bus capacitors in the power modules before switching on themedium voltage is effectively implemented in one embodiment of thepresent invention. During pre-charging, the auxiliary power sources AuxPowers can provide a DC input to the pre-charging units via the charginginput ends. After completion of pre-charging, the medium voltagealternating current can provide an AC input to the power modules via thepower input ends.

Source of the auxiliary power source Aux Power in the embodiment of thepresent invention will be described in detail below with reference toFIGS. 5A to 5C.

In the embodiment shown in FIG. 3A, the plurality of auxiliary powersources Aux Powers 1 to N may be implemented as an Aux Power module(APM) as shown in FIG. 5A or FIG. 5B. The Aux Power module APM will beelectrically connected to the charging input end of each of theplurality of power modules 1 to N.

As shown in FIG. 5A, the Aux Power module APM may be implemented as anAC/DC module. For example, the Aux Power module APM may include atransformer TX, a plurality of rectifier diodes D₁ to D₄, and arectifying capacitor C. An input V₁ of the Aux Power module APM mayreceive a medium-voltage AC input of the power conversion system or anAC input of commercial power (e.g., 220V/50 Hz, 110V/60 Hz, etc.). Anoutput V_(d) of the Aux Power module APM is direct current. For example,power from the medium-voltage AC power source in FIG. 3A may be inputthe Aux Power module APM directly, stepped down by the transformer TX,regulated by the rectifier diodes D₁ to D₄ and the rectifying capacitorC, and then output to a corresponding one of the pre-charging units 1 toN directly as an output voltage V_(d).

Alternatively, as shown in FIG. 5B, the Aux Power module APM may beimplemented as a DC/DC module. For example, the Aux Power module APM mayinclude a conversion circuit and a filter capacitor C. An input V_(in)of the Aux Power module APM may be a DC input. For example, the AuxPower module APM may be electrically connected to a storage batterydirectly. An output V_(d) of the Aux Power module APM may be directcurrent. For example, when the power conversion system is electricallyconnected to an external energy storage battery or a storage batteryincluded in the power conversion system as a backup power source, theAux Power module APM may receive power from the storage battery, thevoltage of which is then converted through the DC/DC conversion circuit,such as Buck/Boost, and output to a corresponding one of thepre-charging units 1 to N as an output voltage V_(d).

In another embodiment, the plurality of auxiliary power sources AuxPowers 1 to N shown in FIG. 3A may be implemented as the Aux Powermodule APM shown in FIG. 5C. As shown in FIG. 5C, the Aux Power moduleAPM includes an Aux power isolation unit APIU. The Aux power isolationunit APIU includes a plurality of secondary circuits that areelectrically connected to the charging input ends of the plurality ofpower modules 1 to N, respectively. More specifically, in FIG. 5C, theAux Power module APM includes a full-bridge LC resonant circuit, atwo-stage transformer which contains two stages (1:N1 and 1:N2), andthree secondary circuits SC₁ to SC₃ which may be implemented asrectifier circuits. Furthermore, the full-bridge LC resonant circuitincludes a capacitor C₁, transistors S₁ to S₄, an inductor L_(r), acapacitor C_(r) and the like. The secondary circuit SC₁ includes diodesD₁ to D₄, a diode VD₁, a transistor VT₁, and a capacitor C₂. Thesecondary circuit SC₂ includes diodes D₅ to D₈, a diode VD₂, atransistor VT₂, and a capacitor C₃. The secondary circuit SC₃ includesdiodes D₉ to D₁₂, a diode VD₃, a transistor VT₃, and a capacitor C₄. Aninput voltage V_(in) of the Aux Power module APM may be obtained byconverting the commercial power through a switching power supply, Theinput voltage V_(in) is converted through the full-bridge LC resonantcircuit, and is then voltage-converted through the two-stagetransformer. Outputs from the three secondary sides of the transformerare regulated through the rectifier circuits (the secondary circuits SC₁to SC₃), respectively, to obtain DC outputs V_(d1) to V_(d3). Theoutputs V_(d1) to V_(d3) may provide DC inputs for three pre-chargingunits, respectively. Of course, it is understood that although onlythree secondary circuits SC₁ to SC₃ are shown in FIG. 5C, in otherembodiments, a different number of the secondary circuits may be used.

In other embodiments, the Aux Power module APM may be implemented as anAC/DC module. An input V_(in) of the Aux Power module APM may beelectrically connected to a medium-voltage AC power source or acommercial power. Alternatively, the Aux Power module APM may beimplemented as a DC/DC module. In this embodiment, the input V_(in) ofthe Aux Power module APM may be electrically connected to a storagebattery.

It is understood that the source of the auxiliary power source Aux Powerin the embodiments of the present invention is not limited to the aboveembodiments. Also, the circuit design for the Aux Power module APM isnot limited to the above embodiments, either, and may be designed andmodified depending on different inputs. The present invention is notlimited thereto.

A plurality of application embodiments of the power conversion system ofthe present invention and a plurality of modifications of thepre-charging units therein will be described in detail below withreference to FIGS. 6-10.

FIG. 6 is a schematic diagram showing a circuit structure of a firstapplication embodiment in which the power conversion system of thepresent invention is applied to a three-level DNPC cascaded system forpre-charging. As shown in FIG. 6, the power conversion system of thefirst application embodiment comprises a power module 1 (PM1) and apower module 2 (PM2).

Each of the power module 1 (PM1) and the power module 2 (PM2) includes apower input end, a charging input end, an AC/DC conversion unit (AC/DC),a DC/DC conversion unit (DC/DC), and a power output end. Moreover, apower input end of the power module 2 (PM2) and a power input end of thepower module 1 (PM1) are connected in series and then electricalconnection with a medium-voltage AC power source.

Further, each of the power module 1 (PM1) and the power module 2 (PM2)includes two DC-Bus capacitors connected in series. For example, thepower module 1 (PM1) includes DC-Bus capacitors C₁₁ and C₁₂ connected inseries. The power module 2 (PM2) includes DC-Bus capacitors C₂₁ and C₂₂connected in series. In each of the power module 1 (PM1) and the powermodule 2 (PM2), the AC/DC conversion unit (AC/DC) is electricallyconnected to the power input end, and the two DC-Bus capacitorsconnected in series. In each of the power module 1 (PM1) and the powermodule 2 (PM2), the DC/DC conversion unit (DC/DC) is electricallyconnected to the power output end, and the two DC-Bus capacitorsconnected in series.

Further, each of the power module 1 (PM1) and the power module 2 (PM2)includes a pre-charging unit. For example, the power module 1 (PM1)includes a pre-charging unit 1 (PU1). The power module 2 (PM2) includesa pre-charging unit 2 (PU2). The pre-charging unit in each of the powermodule 1 (PM1) and the power module 2 (PM2) includes a primary circuit,an isolation transformer, and two secondary circuits. In eachpre-charging unit, the primary circuit is electrically connected to thecharging input end of the power module, for receiving direct currentV_(d). Further, the two secondary circuits are electrically connected tothe two DC-Bus capacitors in parallel in a one-to-one manner, in whichthe two DC-Bus capacitors are connected in series, so as to pre-chargethe corresponding DC-Bus capacitors, respectively.

In the first application embodiment, the AC/DC conversion units (AC/DC)in each of the power module 1 (PM1) and the power module 2 (PM2) mayinclude, for example, a three-level conversion circuit. Each of thethree-level conversion circuits may include two bridge arms connected inparallel. Each bridge arm may include four transistors connected inseries. The two transistors in the middle of each bridge arm areconnected in parallel to two clamp diodes which are connected in series,as shown in FIG. 6.

In the first application embodiment, the DC/DC conversion units (DC/DC)in each of the power module 1 (PM1) and the power module 2 (PM2) mayinclude, for example, an LLC resonant isolation circuit. For example,the LLC resonant isolation circuit may include an LLC resonant unit anda power isolation unit. The power isolation unit may be implemented asan integrated transformer, for example. As shown in the DC/DC part ofFIG. 6, the power isolation unit is typically implemented as atransformer. The LLC resonant unit may include an LLC resonant tank. TheLLC resonant tank may include a resonant inductor L_(r) and a resonantcapacitor C_(r).

In the first application embodiment, the circuit structure of thepre-charging unit 1 (PU1) in the power module 1 (PM1) may bealternatively designed as shown in FIG. 7. In FIG. 7, the pre-chargingunit 1 (PU1) may be implemented in a full-bridge LC resonant circuittopology, and includes a primary circuit Cir-p, an isolation transformerIso, and two secondary circuits Cir-s-1 and Cir-s-2. For example, theprimary circuit Cir-p may be implemented as a full-bridge LC resonantcircuit which includes transistors S₁ to S₄, a capacitor C_(s), aresonant inductor L_(r), and a resonant capacitor C_(r). For example,the isolation transformer Iso may be implemented as a step-uptransformer TX. The secondary circuit Cir-s-1 is electrically connectedin parallel to both ends of the DC-Bus capacitor C₁₁, and, for example,may be implemented as a rectifier circuit including rectifyingcapacitors C₁, C₂ and rectifier diodes D₁, D₂. The secondary circuitCir-s-2 is electrically connected in parallel to both ends of the DC-Buscapacitor C₁₂, and, for example, may be implemented as a rectifiercircuit including rectifying capacitors C₃, C₄, and rectifier diodes D₃,D₄. As such, the pre-charging unit 1 (PU1) may receive power from theinput Vd of the charging input end. Then, a square wave is generated inthe primary circuit Cir-p, which is then stepped-up through theisolation transformer Iso, voltage-doubled and regulated through thesecondary circuits Cir-s-1 and Cir-s-2, and then supplied forprecharging the DC-Bus capacitors C₁₁ and C₁₂, respectively. The circuitstructure of the pre-charging unit 2 (PU2) of the power module 2 (PM2)may be similar to that shown in FIG. 7. Therefore, detailed descriptionsare omitted here. In the first application embodiment, the isolationtransformer Iso and the two secondary circuits Cir-s-1 and Cir-s-2 forma pre-charging isolation unit.

Moreover, it is understood that in other embodiments of the presentinvention, the pre-charging unit in each of the power module 1 (PM1) andthe power module 2 (PM2) is not limited to the structure shown in FIG.7, and may be combined and modified depending on the form of thepre-charging units and the main power circuits (such as the powerconversion units) in the power modules. Some examples of thepre-charging unit will be described below. However, the presentinvention is not limited thereto.

For example, each of the secondary circuits Cir-s-1 and Cir-s-2 in FIG.7 may alternatively be implemented as a voltage-doubled rectifiercircuit including two rectifier diodes connected in series and arectifier capacitor. Alternatively, each of the secondary circuitsCir-s-1 and Cir-s-2 in FIG. 7 may be implemented as a single voltagerectifier circuit including four rectifier diodes. Alternatively, theprimary circuit Cir-p in FIG. 7 may be implemented as an LLC resonantcircuit. The above-mentioned modifications of the pre-charging unit mayalso be applicable to power modules including two or more DC-Buscapacitors therein, and the present application is not limited thereto.

For example, when there is only one DC bus in each power module, thenumber of the secondary circuits in FIG. 7 or the above-mentionedmodifications of the pre-charging unit can be designed as oneaccordingly. However, when there are three DC-Bus capacitors connectedin series in each power module, the number of the secondary circuits inFIG. 7 and the above-mentioned modifications of the pre-charging unitmay be designed as three accordingly.

Furthermore, it is understood that in other embodiments of the presentinvention, the main power circuit (such as the power conversion unit) ineach of the power module 1 (PM1) and the power module 2 (PM2) may alsobe modified. For example, in FIG. 6, in each of the power module 1 (PM1)and the power module 2 (PM2), the DC/DC conversion units (DC/DC) in thepower conversion unit may be implemented in a series-connectedhalf-bridge LLC circuit topology which utilizes an integratedtransformer and outputs in a full-bridge structure. In otherembodiments, the DC/DC converter unit (DC/DC) may alternatively beimplemented in a DNPC LLC circuit topology which utilizes an integratedtransformer and outputs in a full-bridge structure. The AC/DC circuitand/or the DC/DC circuit in the power module may be implemented in anycircuit topologies that may realize the functions, and the presentinvention is not limited thereto.

For example, in FIG. 6, in each of the power modules 1 (PM1) and thepower module 2 (PM2), the power conversion unit includes two DC-Buscapacitors connected in series. Each of the DC-Bus capacitors includes aplurality of groups of capacitors connected in parallel. A group ofcapacitors may be connected in series. However, the present applicationis not limited thereto. In other embodiments, a plurality of DC-Buscapacitors may be connected in parallel. Alternatively, the number ofDC-Bus capacitors in each power module may be one or three or more.Further, the three or more DC-Bus capacitors may be connected in seriesor may be connected in other manners.

FIG. 8 is a schematic diagram showing a circuit structure of a secondapplication embodiment in which the power conversion system of thepresent invention is applied to a full-bridge cascaded system forpre-charging. In the second application embodiment, each of the powermodule 1 (PM1) and the power module 2 (PM2) includes three cascadedpower conversion units, each of which includes a DC-Bus capacitor. Thatis, the power module 1 (PM1) includes three groups of DC-Bus capacitorsC₁₁ to C₁₃, while the power module 2 (PM2) includes three groups ofDC-Bus capacitors C₂₁ to C₂₃. Moreover, the pre-charging isolation unitin each of the pre-charging unit 1 (PU1) of the power module 1 (PM1) andthe pre-charging unit 2 (PU2) of the power module 2 (PM2) includes threesecondary circuits correspondingly. The secondary circuits areelectrically connected in parallel to the DC-Bus capacitors in the powerconversion units, respectively.

It is understood that, in other modifications, each of the power module1 (PM1) and the power module 2 (PM2) may alternatively include twocascaded power conversion units, each of which includes one or moreDC-Bus capacitors. Correspondingly, the pre-charging isolation unit ineach of the pre-charging unit 1 (PU1) of the power module 1 (PM1) andthe pre-charging unit 2 (PU2) of the power module 2 (PM2) may includetwo secondary circuits, for pre-charging the DC-Bus capacitors in thetwo cascaded power conversion units, respectively. The number of thecascaded power conversion units and the number of the DC-Bus capacitorscan be varied as necessary, and the present invention is not limitedthereto.

FIG. 9 is a schematic diagram showing a circuit structure of a thirdapplication embodiment in which the power conversion system of thepresent invention is applied to a DC/DC full-bridge system forpre-charging. In the third application embodiment, each of the powermodule 1 (PM1) and the power module 2 (PM2) includes a power conversionunit which may be implemented in a full-bridge LLC topology circuitstructure, for example. The power module 1 (PM1) and the power module 2(PM2) further includes DC-Bus capacitors C₁₁ and C₂₁, respectively.Correspondingly, the pre-charging isolation unit in each of thepre-charging unit 1 (PU1) of the power module 1 (PM1) and thepre-charging unit 2 (PU2) of the power module 2 (PM2) includes asecondary circuit. The secondary circuit is electrically connected inparallel to the DC-Bus capacitor directly, so as to pre-charge theDC-Bus capacitor directly.

FIG. 10 is a schematic diagram showing a circuit structure of a fourthapplication embodiment in which the power conversion system of thepresent invention is applied to a three-level DNPC cascaded system forpre-charging. The fourth application embodiment differs from the firstapplication embodiment shown in FIG. 6 in that the power conversion unitin each of the power module 1 (PM1) and the power module 2 (PM2)includes an AC/DC conversion unit (AC/DC), instead of a DC/DC conversionunit (DC/DC).

FIG. 11 is a flow chart showing a method for pre-charging DC-Buscapacitors in a medium-voltage power conversion system according to oneembodiment of the present invention.

As shown in FIG. 11, the method includes the following steps:

a step of converting direct current input from charging input ends, viapre-charging units in a plurality of power modules when themedium-voltage power conversion system is activated, to pre-charge eachDC-Bus capacitor in the plurality of power modules; and

a step of stopping the operations of the pre-charging units after avoltage of the DC-Bus capacitor reaches a threshold, and convertingalternating current input from power input ends, via power conversionunits in the plurality of the power modules, to output electrical energyat power output ends.

Moreover, with the pre-charging units, a certain power module may becontrolled to be cut off, so that when one of the plurality of powermodules breaks down or the load of the power conversion system is light,only one power module may be cut off while the other power modulesoperate normally. Hereafter the corresponding operation will bedescribed in detail.

FIG. 12 is a preferred embodiment of an AC/DC conversion unit in a powerconversion unit in each of the power modules of FIG. 3A. The AC/DCconversion unit may comprise a bridge rectifier circuit which includestwo bridge arms connected in parallel, each bridge arm including twotransistors connected in series. Each of the transistors has a diode,which may be either a body diode of the transistor or an independentdiode. When one of the plurality of power modules breaks down, drivingsignals for the transistors of that power module are cut off and thecorresponding pre-charging unit starts to charge the DC-Bus capacitor Cof that power module continuously or intermittently to a voltage higherthan a voltage V′_(in) at the power input end, so that the diodes inthat bridge rectifier circuit are off, thereby no current flowingthrough the AC/DC conversion unit. As such, when a certain phase (phaseA, B, or C) breaks down, only that phase is cut off while the other twophases operate normally to supply the load. In a similar manner, whenthe load of the power conversion system is light, i.e., less than 40% ofa full-load power, the pre-charging unit for a certain power module maystart to charge the DC-Bus capacitor C of the corresponding power modulecontinuously or intermittently to a voltage higher than a voltageV′_(in) at the power input end, so that no current flows through theAC/DC conversion unit in that power module. As such, when the load ofthe power conversion system is light, a certain power module is cut offto realize low power output.

It is understood that, in other modifications, the AC/DC conversion unitmay further comprise a DC/DC converter, as shown in FIG. 13. The DC/DCconverter may be a buck converter or a boost converter. An output of thebridge rectifier circuit is connected to a first DC-Bus capacitor C₁ andan input of the DC/DC converter, and an output of the DC/DC converter isconnected to a second DC-Bus capacitor C₂. When one of the plurality ofpower modules breaks down or the load of the power conversion system islight, the pre-charging unit for a certain power module starts to chargethe second DC-Bus capacitor C₂ of that power module so that a voltage onthe first DC-Bus capacitor C₁ is higher than a voltage V′_(in) at thepower input end, thereby no current flowing through the AC/DC conversionunit. With the addition of a DC/DC converter in the AC/DC conversionunit, a voltage on the second DC-Bus capacitor C₂ may be designedflexibly, with the input voltage of the subsequent DC/DC conversioncircuit being taken into consideration at the same time.

FIG. 14 is another preferred embodiment of an AC/DC conversion unit in apower conversion unit in each of the power modules of FIG. 3A. The AC/DCconversion unit may include a three-level conversion circuit whichincludes two bridge arms connected in parallel. Each bridge arm mayinclude four transistors connected in series, and each of thetransistors has a diode, which may be either a body diode of thetransistor or an independent diode. The two transistors in the middle ofeach bridge arm are connected in parallel to two clamp diodes which areconnected in series. When one of the plurality of power modules breaksdown, driving signals for the transistors of that power module are cutoff and the corresponding pre-charging unit starts to charge the DC-Buscapacitors C₁, C₂ of that power module continuously or intermittently toa voltage higher than a voltage V′_(in) at the power input end, so thatthe diodes in that three-level conversion circuit are off, thereby nocurrent flowing through the AC/DC conversion unit. As such, when acertain phase (phase A, B, or C) breaks down, only that phase is cut offwhile the other two phases operate normally to supply the load. In asimilar manner, when the load of the power conversion system is light,i.e., less than 40% of a full-load power, the pre-charging unit for acertain power module may start to charge the DC-Bus capacitor C₁, C₂ ofthe corresponding power module continuously or intermittently to avoltage higher than a voltage V′_(in) at the power input end, so that nocurrent flows through the AC/DC conversion unit in that power module. Assuch, when the load of the power conversion system is light, a certainpower module is cut off to realize low power output. By adopting such aDNPC bridge rectifier circuit, the number of cascaded power units in thesystem can be reduced with devices of same voltage level.

In summary, in one embodiment of the present invention, each powermodule is provided with a pre-charging unit which is electricallyconnecting to a DC-Bus capacitor in the power module, so as to alloweach power module to complete the pre-charging independently. Further,when the pre-charging unit in an individual power module does not work,the entire system will not fail as a result. Moreover, since the mainloop current does not flow through the pre-charging unit in each powermodule, lower power consumption may be achieved within the same chargingperiod. Further, the pre-charging unit may be implemented in a smallersize.

Preferably, in one embodiment of the present invention, each powermodule may be hot plugged by modularizing respective units in the powermodules, for example, by integrating the pre-charging unit, the powerconversion unit, the controller, the voltage sampling unit, and the likein each power module onto a power board.

Preferably, in one embodiment of the present invention, an auxiliarypower module is used for supplying auxiliary power to each pre-chargingunit in each power module independently. That is, the power may besupplied without using the medium-voltage input of the system directly,thereby helping to resume the charging from a break point, allowing tocontrol the charging more easily, and avoiding risk of unbalance incapacitor voltage of the DC-Bus capacitors in each power module.Moreover, in one embodiment of the present invention, the DC-Buscapacitors in each power module may be pre-charged effectively before aswitch for the medium voltage is closed.

Preferably, in some embodiments of the present invention, by starting tocharge the DC-Bus capacitor of a certain power module by use of thepre-charging unit to a voltage higher than a voltage at the power inputend, no current may flow through that power module. As such, when acertain power module breaks down, only that power module is cut offwhile the other two power modules operate normally to supply the load;or, when the load of the power conversion system is light, a certainpower module is cut off to realize low power output.

While the present invention has been disclosed in the aboveimplementations, it is not intended to limit the present invention, andvarious modifications and retouches may be made by those skilled in theart without departing from the spirit and scope of the presentinvention. The scope of protection of the present invention therefore issubject to the scope defined by the appended claims.

What is claimed is:
 1. A power conversion system connecting to a load,comprising: a plurality of power modules, each including: a power inputend; a charging input end; a power output end; at least one powerconversion unit, each of the power conversion unit including an AC/DCconversion unit and at least one DC-Bus capacitor and being electricallyconnected to the power input end and the power output end, wherein theAC/DC conversion unit is electrically connected between the power inputend and the DC-Bus capacitor; and a pre-charging unit electricallyconnected to the charging input end for receiving direct current andelectrically connected to the DC-Bus capacitor, the pre-charging unitstarting to charge the DC-Bus capacitor of one of the plurality of powermodules when the one of the plurality of power modules breaks down orthe load of the power conversion system is light so that no currentflows through the AC/DC conversion unit, wherein, the power input endsof the plurality of power modules are connected in series and thenelectrically connected to an AC power source, and the power output endsof the plurality of power modules are connected in parallel; wherein theAC/DC conversion unit comprises a bridge rectifier circuit whichincludes two bridge arms connected in parallel, each bridge armincluding two transistors connected in series, wherein each of thetransistors has a diode, and wherein driving signals for the transistorsof the one of the plurality of power modules are cut off when the one ofthe plurality of power modules breaks down or the load of the powerconversion system is light; wherein the pre-charging unit charges theDC-Bus capacitor to a voltage higher than a voltage at the power inputend so that the diode is off.
 2. The power conversion system accordingto claim 1, the diode is a body diode of the transistor or anindependent diode.
 3. The power conversion system according to claim 1,wherein the pre-charging unit includes a pre-charging isolation unitwhich includes a plurality of secondary circuits.
 4. The powerconversion system according to claim 3, wherein each of the powerconversion unit includes a plurality of the DC-Bus capacitors connectedin series, and the secondary circuits of the pre-charging isolation unitare electrically connected to the DC-Bus capacitors connected in series,respectively.
 5. The power conversion system according to claim 3,wherein each of the power modules includes a plurality of the powerconversion units cascaded, and the secondary circuits of thepre-charging isolation unit are electrically connected to the DC-Buscapacitors in the cascaded power conversion units, respectively.
 6. Thepower conversion system according to claim 1, further comprising: aplurality of auxiliary power modules electrically connected to thecharging input ends of the power modules, respectively, wherein theauxiliary power modules are DC/DC modules and are electrically connectedto a storage battery, or the auxiliary power modules are AC/DC modulesand are electrically connected to the AC power source.
 7. The powerconversion system according to claim 1, further comprising: an auxiliarypower module including an auxiliary power isolation unit, the auxiliarypower isolation unit including a plurality of secondary circuits,wherein the secondary circuits of the auxiliary power isolation unit areelectrically connected to the charging input ends of the plurality ofpower modules, respectively.
 8. The power conversion system according toclaim 7, wherein the auxiliary power module is an AC/DC module, and theinput end of the auxiliary power module is electrically connected to theAC power source or a commercial power; or the auxiliary power module isa DC/DC module, and the input end of the auxiliary power module iselectrically connected to a storage battery.
 9. The power conversionsystem according to claim 1, wherein the pre-charging unit is configuredto pre-charge the DC-Bus capacitor before the power conversion isperformed through switching operations of the power modules and stop thepre-charging after a voltage of the DC-Bus capacitor reaches athreshold.
 10. The power conversion system according to claim 1, whereinthe pre-charging unit starts to charge the DC-Bus capacitor continuouslyor intermittently.
 11. The power conversion system according to claim 1,wherein the AC/DC conversion unit further comprises a DC/DC converter,an output of the bridge rectifier circuit being connected to asupplemental DC-Bus capacitor and an input of the DC/DC converter, andan output of the DC/DC converter being connected to the DC-Buscapacitor, and wherein the pre-charging unit starts to charge the DC-Buscapacitor so that a voltage on the supplemental DC-Bus capacitor ishigher than a voltage at the power input end, thereby the diode beingoff.
 12. The power conversion system according to claim 1, wherein theAC/DC conversion unit comprises a three-level conversion circuit,wherein each of the three-level conversion circuits includes two bridgearms connected in parallel, each bridge arm including four transistorsconnected in series, each of the transistors having a diode, and the twotransistors in the middle of each bridge arm being connected in parallelto two clamp diodes which are connected in series, and wherein thepre-charging unit charges the DC-Bus capacitor to a voltage higher thana voltage at the power input end so that the diode is off.
 13. The powerconversion system according to claim 12, wherein driving signals for thetransistors of the one of the plurality of power modules are cut offwhen the one of the plurality of power modules breaks down or the loadof the power conversion system is light.